1825.] Variation of the Mariner's Compass, 6 



tienced the same but about four years later. I find other 

 instances where the variation amounted to about 7° in 68 years, 

 and afterwards in the very same place to only 15 minutes in the 

 whole space of 60 years. These contradictions must probably 

 be ascribed to the incorrectness of the instruments, and the 

 influence of local attraction. Both these inconveniences may 

 now be considered as almost totally removed by the great im- 

 provements in the construction of compasses, and by Dr. Bar- 

 low's apparatus to counteract the influence of iron. The best 

 instruments, however, are still found to have some small defects, 

 which must be ascertained before we can use them with advan- 

 tage. I see in Capt. Parry's voyage in the years 1819 and 1820, 

 that at several places the variation was observed with four com- 

 passes, all of the best workmanship, and yet they all disagreed, 

 and consequently three of them at least, if not all, must have 

 been wrong. But this is no objection to their utihty, for if after 

 the experiment has been repeated with all of them in different 

 parts of the world, their differences are found to bear always the 

 same proportion, it is certain that we may then conclude from 

 the variation of any single one, what would be that of either of 

 the remaining three ; and if all the compasses made use of for 

 discovery had first been tried in this manner, we might then be 

 able to reduce all the observations to one common standard, in 

 the same manner as we may acquire the very same notions of the 

 state of the atmosphere with barometers and thermometers of 

 different constructions. I have no doubt, but that some regard 

 must be paid to the temperature of the air, besides the respective 

 situation of the sun, moon, and the earth. The latitude of the 

 magnetic pole might also be sometimes increased or diminished 

 according to the decUnations of these objects, and it remains to 

 be determined, whether it revolves in a circle, or in an ellipsis. 

 As I have neither instruments, nor any of the resources requisite 

 for the investigation of so intricate a subject, I must resign the 

 honour of deciding these questions to those, whose happier cir 

 cumstances allow them to indulge themselves in the daily con 

 templation and admiration of the wonders of the creation. 



After having conceived the idea of estabHshing my theory on 

 the revolution of the heavens, I chose, to prove it, the observa- 

 tion made in London by Dr. Gilpin in 1812, the only one of 

 which I had also the dip. The result is, as I have mentioned 

 before, that the annual progress of the magnetic pole round the 

 pole of the earth is nearly in direct proportion as annual pro- 

 gress of the nodes of Venus to the annual progress of the nodes 

 of the earth. It would have contributed greatly to my satisfac- 

 tion, if I had been able to obtain the same result from two or 

 more good observations made at the same time in different lati- 

 tudes and longitudes, but these I could not procure ; for those, 

 which I might have taken from several voyages, were either not 



